In the majority of applications printing proceeds by pressure contact of an ink-loaded printing form with an ink-receiving material which is usually plain paper. The most frequently used impact printing technique is known as lithographic printing based on the selective acceptance of oleophilic ink on a suitable receptor. In recent times however so-called non-impact printing systems have replaced classical pressure-contact printing to some extent for specific applications. A survey is given e.g. in the book “Principles of Non Impact Printing” by Jerome L. Johnson (1986), Palatino Press, Irvine, Calif. 92715, USA.
Among non-impact printing techniques ink-jet printing has become a popular technique because of its simplicity, convenience and low cost. Especially in those instances where a limited edition of the printed matter is needed ink-jet printing has become a technology of choice. A recent survey on progress and trends in ink-jet printing technology is given by Hue P. Le in Journal of Imaging Science and Technology Vol. 42 (1), January/February 1998.
In ink-jet printing tiny drops of ink fluid are projected directly onto an ink receptor surface without physical contact between the printing device and the receptor. The printing device stores the printing data electronically and controls a mechanism for ejecting the drops image-wise. Printing is accomplished by moving the print head across the paper or vice versa. Early patents on ink-jet printers include U.S. Pat. Nos. 3,739,393, 3,805,273 and 3,891,121.
The jetting of the ink droplets can be performed in several different ways. In a first type of process a continuous droplet stream is created by applying a pressure wave pattern. This process is known as continuous ink-jet printing. In a first embodiment the droplet stream is divided into droplets that are electrostatically charged, deflected and recollected, and into droplets that remain uncharged, continue their way undeflected, and form the image. Alternatively, the charged deflected stream forms the image and the uncharged undeflected jet is recollected. In this variant of continuous ink-jet printing several jets are deflected to a different degree and thus record the image (multideflection system). According to a second process the ink droplets can be created “on demand” (“DOD” or “drop on demand” method) whereby the printing device ejects the droplets only when they are used in imaging on a receiver thereby avoiding the complexity of drop charging, deflection hardware, and ink recollection. In drop-on-demand the ink droplet can be formed by means of a pressure wave created by a mechanical motion of a piezoelectric transducer (so-called “piezo method”), or by means of discrete thermal pushes (so-called “bubble jet” method, or “thermal jet” method).
Ink compositions for ink-jet typically include following ingredients: dyes or pigments, water and/or organic solvents, humectants such as glycols, detergents, thickeners, polymeric binders, preservatives, etc. It will be readily understood that the optimal composition of such an ink is dependent on the ink-jetting method used and on the nature of the substrate to be printed. The ink compositions can be roughly divided in:                water based; the drying mechanism involves absorption, penetration and evaporation;        oil based; the drying involves absorption and penetration;        solvent based; the drying mechanism involves primarely evaporation;        hot melt or phase change: the ink vehicle is liquid at the ejection temperature but solid at room temperature; drying is replaced by solidification;        UV-curable; drying is replaced by polymerization.        
U.S. Pat. No. 4,877,686 discloses a recording sheet for use in connection with inkjet printing comprising an opaque base sheet and a surface coating on said base sheet, said surface coating comprising a polyhydroxylic polymeric binder with the hydroxyl groups in the cis position, a substantial portion of said binder having been gelled with a gelling agent selected from the group consisting of boric acid, derivatives of boric acid, and mixtures thereof, and a filler component having high absorption capacity, said binder being present in an amount of from about 10 to 100 percent by weight of the amount of said filler, whereby said filler primarily acts as the ink receptor in said ink-jet printing and the shape, size and uniformity of dots of said ink as applied to said recording sheet may be substantially improved thereby.
US 2001/014381 A1 discloses an ink-jet recording material comprising a support, and an ink-receptive layer containing fumed silica having an average primary particle size of 3 nm to 30 nm provided on the support, wherein said ink-receptive layer contains a cationic compound and at least one compound selected from the group consisting of a sulfur-containing compound having no mercapto group, an amine compound, an amino compound and a saccharide, and a pH of the surface of the ink-jet recording material is 3 to 6.
EP 888,904 A1 discloses an ink-jet recording method comprising: recording on an ink-jet recording sheet comprising a non-water-absorbing support and provided thereon an ink absorbing layer containing polyvinyl alcohol, fine inorganic particles and boric acid or its salt, using an ink-jet recording apparatus and a water-based recording liquid containing a high boiling solvent containing a hydroxy group, wherein the following requirements (1) and (2) are met: (1) 0.05≦X/Y≦0.5 and (2) Z/Y≦4, wherein X is an amount of boric acid or its salt contained in the ink absorbing layer of the recording sheet, expressed in mmol/m; Y is an amount of a hydroxy group contained in polyvinyl alcohol contained in the ink absorbing layer of the recording sheet, expressed in mmol/m; and Z is a maximum amount of the hydroxy group contained in the high boiling solvent contained in a unit area when recorded on the recording sheet at a maximum ejecting amount of the water-based recording liquid, expressed in mmol/m2.
It is known that the ink-receiving layers in ink-jet recording materials must meet different stringent requirements:                the ink-receiving layer should have a high ink absorbing capacity, so that the dots will not flow out and will not be expanded more than is necessary to obtain a high optical density;        the ink-receiving layer should have a high ink absorbing speed (short ink drying time) so that the ink droplets will not feather if smeared immediately after applying;        the ink dots that are applied to the ink-receiving layer should be substantially round in shape and smooth at their peripheries. The dot diameter must be constant and accurately controlled;        the receiving layer must be readily wetted so that there is no “puddling”, i.e. coalescence of adjacent ink dots, and an earlier absorbed ink drop should not show any “bleeding”, i.e. overlap with neighbouring or later placed dots;        transparent ink-jet recording elements must have a low haze-value and be excellent in transmittance properties;        after being printed the image must have a good resistance regarding water-fastness, light-fastness, and good endurance under severe conditions of temperature and humidity;        the ink-jet recording element may not show any curl or sticky behaviour if stacked before or after being printed; and        the ink-jet recording element must be able to move smoothly through different types of printers.All these properties are often in a relation of trade-off. It is difficult to satisfy them all at the same time.        
In order to obtain images showing high gloss, high color densities and fast drying it is desirable that the ink receiving layer has a relative high coating weight and a high pigment/binder ratio. However, such a high pigment/binder ratio tends to deteriorate the mechanical strength of the ink receiving layer, in particular when a flexible support is used, which is often visible as microcracks. It is strongly desired to find measures to avoid this cracking while retaining the other good image properties.